(Today's guest post is by Joel Poindexter. If you would like to guest write for us, please check out our guest post guidelines.)

“The surge worked.” A popular phrase at the conclusion of the final troop surge, it has again made its way into the discussion of U.S. operations in Iraq. As the Islamic State of Iraq and Syria (ISIS) has risen to power, politicians have taken to reviving this talking point. Depending on one’s definition of success, it’s debatable as to whether the surge did in fact work.

Officially “The New Way Forward,” by its own standards it did not achieve the stated goals. A series of benchmarks were established, which the Shi’a-backed Iraqi government was expected to achieve. As of the end of Operation Iraqi Freedom, in December, 2011, few of the benchmarks had been satisfactorily met, in spite of the rhetoric of success. Nevertheless, there is at least one sense in which we can say the surge worked.

Most who are familiar with the surge no doubt associate it with the increase in U.S. troops in 2007. Five additional brigades were deployed, and most units had their rotations extended. This was meant to provide sufficient security in the capitol, to facilitate those benchmarks. What few likely understand is that a major component involved the establishment of a para-military force almost entirely made up of Sunni forces.

American commanders spent millions of dollars financing groups that went by Sons of Iraq (SOI) or Concerned Local Citizens (CLC). Many were formerly employed by Sunni militias, including al-Qaeda in Iraq (AQI) and the Islamic State of Iraq (ISI – an AQI affiliate), and some were known to have been members of AQI.

In late 2007 I was deployed to a small FOB south of Baghdad, and our battalion spent a lot of time (and money) hiring and managing SOI. The relationship was contentious, as neither group trusted the other. In our fifteen months there were several incidents involving “green on blue” gunfights, and reciprocal threats of IED attacks and airstrikes were exchanged.

SOI routinely complained of not being paid, despite monthly cash payments to village sheikhs in the hundreds of thousands of dollars. Some intelligence suggested SOI leadership (AQI) was diverting payments to build reserves. These would be necessary when the U.S. military withdrew its forces, or if the Iraqi government failed to incorporate the SOI into the Iraqi security forces, according to the benchmarks.

No doubt the SOI programs helped insulate Sunni militias, and sustained them through the end of the U.S. occupation. But as significant as this aid was to ISIS, the element of the surge that really came to help the organization was the continued support of the Iraqi government.

By escalating the war in Baghdad, the U.S. military helped the Iraqi army (IA) and police (IP) all but complete the sectarian cleansing of the largest city. Various Shi’ite militia groups were represented in the IA and IP, both of which served the interests of the Badr Corps , the Jaish al-Mahdi (JAM), and other groups allied or affiliated with the United Iraqi Alliance. The UIA controlled the country’s government, mainly for the benefit of the Shi’ite population.

While the U.S. fought both Shi’a and Sunni groups throughout the occupation, this period – and the surge effort in particular – focused mainly on routing Sunni forces intent on destabilizing the Iraqi government. This was supposed to provide the breathing room necessary to make some legislative reforms and begin reconciling the rival sectarian groups under one cooperative government.

But so long as U.S. commanders were supporting the Iraqi government, the Shi’a had no incentive to reconcile with the Sunni. Dr. Michael Izady’s work on the Gulf 2000 Project, through Columbia University, demonstrates this visually. In 2003 Shi’ites had a majority in Baghdad, but most of the city’s neighborhoods were mixed. By early 2007 few were home to both groups, and most the territory was controlled by Shi’a. At mid-2008 there were clear lines separating neighborhoods. When we flew over Baghdad that summer, it was easy to see how thousands of concrete barriers had effectively reduced the “city of peace” to sectarian ghettos.

So divided was the country following U.S. withdrawal, that despite the ruthlessness of ISIS, many Sunnis see the Caliph as the lesser evil between it and the Shi’a death squads of the national government. Had the U.S. not fought on behalf of the Shi’a in Baghdad, the government would have been forced to reconcile, eliminating much of the support for ISIS.

(Note: This argument assumes the invasion and occupation as given, and of course both were significant in leading to the Islamic State. The support of rebel groups in Syria (Jabhat al-Nusra, the Northern Storm Brigade, and others) also cannot be discounted.)

Joel Poindexter was an infantryman and intelligence analyst in the US Army from 2003-2009. He served in Baghdad in 2005, and Iskandariyah in 2007-2008. Follow him on Twitter.

On Violence is a blog on counter-insurgency warfare, military and foreign affairs, art, and violence, written by two brothers--one a veteran and the other a pacifist.

The work of On Violence has appeared in The Washington Post, Stars and Stripes, The Small Wars Journal, The New York Times’ "At War" blog, The Los Angeles Times’ Blowback feature, FP.com and Thomas Ricks’ “The Best Defense” blog, Infantry Magazine, and Doonesbury’s “The Sandbox”.